Has anyone compare a J1/V1 + 18.5mm f1.8 against the RX100? With both f1.8 wide open, they both have 1” sensor but the Sony has 20MP verse Nikon has 10MP. Will RX100 has more advantage in low night? If yes, how much it would be (ie. 1/2 stop advantage?)

The comparisons I've seen have been with the V1/J1 and the slower kit lenses and not the 18.5. Based purely on those I'd guess they'd be a wash. The RX100 has been better primarily because the V1 is at f3.5. Use the 18.5 and you've leveled the playing field a bit vs the RX100. The backlit sensor on the II might be better but some folks claim the original is better.

The RX100 is only f1.8 at the wide end but at least you have the option to zoom. With the 18.5 you're zooming with your feet.

If I were doing mostly low light I think I'd go with the RX100. Probably the II for the hotshoe and tilt LCD. It's a nice P&S with better versatility and ease of use. If the if the 6.7mm-13mm zoom was f2.8 or f1.8-4.9 I'd go the other way and pick the V2.